Elastically recoverable silicone splice cover

ABSTRACT

An elastically recoverable elastomeric splice cover suitable for use covering a joint connector in a cable conductor having a central body with two distal opposing geometric cones capped by extended endseals, and formed from three contiguous layers, 
     a) a semiconductive outer shield layer, 
     b) an inner toroidal electrode disposed along a central axis having a length less than that of the body, 
     c) and interposed therebetween, an intermediate insulative layer having a length equal to that of the body and geometric cones. The outer shield layer and electrode are formed from a thermally conductive silicone elastomer, and all of the layers are formed from a silicone elastomer having a tear strength of at least about 15 N/mm, and an elongation of at least about 400%. The insulative layer further includes from about 10% to about 35% of an electrically insulative filler, and has a minimum thermal conductivity of at least about 0.18 W/mK, wherein after 30 alternating load current cycles the connector has maintained a temperature of about 10° C. cooler than the cable conductor beyond the splice.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an elastically recoverable siliconesplice for enclosing a connection or termination of an electrical cable,such splice having improved thermal conductivity and tear strength.

2. Description of the Related Art

Elastically recoverable or recoverable silicone splices are known in theart in various designs.

U.S. Pat. No. 5,171,940 discloses a cold shrink sleeve which isexpanded, and which consists of three layers bonded together whereby thetemporary residual deformation of the innermost layer is less than thecorresponding deformation of the other layers and the modulus ofelasticity of such layer is greater than that of the outer layers. Theexpansion is stated to be in the order of 100% when measuring the innerdiameter prior and after the expansion. The inner layer is formed of amaterial which exhibits a reduced residual deformation on discontinuanceof the applied expansion stress and acts on the outer layers so that thewhole splice can clamp around the smallest cables for which it is used.Upon removal of the support, the inner layer applies a predeterminedradially inward pressure on the cables. The layers are coextruded and/orjointly cross-linked to form the splice.

U.S. Pat. No. 3,717,717, discloses an integral splice which includesthree layers; an inner conductive layer, a middle heat-shrinkableinsulation layer, and an outer semiconductive layer. In order to achievea geometric-capacitive field control, the extremities of the splice areconically shaped.

EPO Appl. 313,978 discloses a single piece radially expandable sleevewhich is substantially uniform cylindrically. The middle and the outerlayer are conventionally structured. The sleeve is formed of siliconeelastomers, rubbers and several portions are filled to provide thedesired conductivity.

U.S. Pat. No. 4,383,131 discloses an enclosure for a shielded cabletermination or splice (joint) comprising a conductive outer layer, aninsulating inner layer, and an optional stress gradient innermost layer.Preferred embodiments are heat-shrinkable; a long list of polymericmaterials includes silicone elastomers. No fillers are disclosed; nor isthermal conductivity of the various polymeric matrices.

U.S. Pat. No. 4,390,745 discloses an enclosure for electrical apparatuscomprising a first hollow sleeve of insulating material around which ispositioned a second sleeve comprising an inner layer of insulatingmaterial and an outer layer of conductive material, e.g., a cableshield. This construction is disclosed to place the majordiscontinuities of the material enclosure wall between layers ofinsulating materials reducing the electrical stress. The first sleevemay have an inner laminate layer of stress grading material. A lengthylist of possible elastomers includes silicone polymers; however, heatrecoverable articles are preferred.

EP 0404 090 A2 discloses an elastomeric covering for connections,including splices, terminations and end-seals, in electrical cablesutilizing elastomeric pre-stretched tubes maintained in a radiallyexpanded state by an inner removable core. A conformable material isinterposed between the prestretched tube and the core whereby, when thecore is removed, the tube conforms to the underlying surface. Theconformable material may be insulative, or conductive, with or withoutdielectric fillers.

However, while silicone materials are sometimes recited as part ofextensive lists, they typically do not possess the thermal conductivityto be used for splices in medium and high voltage cable splices andterminations.

A variety of thermally conductive organosiloxanes are also known in theart for uses as coatings, encapsulants and potting compounds forelectronic devices; coatings for fuser rolls in copying machines, andthe like.

U.S. Pat. No. 4,444,944 discloses a thermally conductive silicone rubbercomposition prepared by mixing 100 parts of an aliphatically unsaturatedpolyorganosiloxane, a polyorganohydrogensiloxane, and from 100 to 500parts alumina powder having an average particle size of 2 μm to 10 μmand an oil absorption greater than 15 mL/g, and a platinum catalyst. Thecured elastomers are disclosed to have tensile strengths of 2.7-4.8 MPa,and elongations of 80-100%. The thermal conductivities range from 1.8 to2.5 cal/cm-sec° C.×10³ (0.75-1.05 W/mK).

U.S. Pat. No. 4,544,696 discloses silicone elastomers which comprise anorganosiloxane having Si-bonded aliphatically unsaturated groups, acatalyst, and from about 30 to about 95 percent of filler; at least 10%of the filler is silicon nitride particles. Other fillers such as metaloxides, aerogels, mica, glass beads and the like are also disclosed.

U.S. Pat. No. 5,011,870 discloses the combination of thermallyconductive fillers with an average particle size of from about 10 toabout 100 microns with submicron aluminum nitride for improved thermalconductivity of organosiloxane compositions.

U.S. Pat. No. 5,352,731 discloses a silicone rubber composition havingthermal conductivity higher than 5×10⁻³ cal/cm-sec° C. (2.09 w/mK). Thecomposition comprises two different types of organopolysiloxanes havingdifferent polymerization degrees, aluminum oxide powder and a curativefor the composition.

However, none of these silicone composition patents disclose electricalcable applications, or use of such materials in splices. The quantitiesof fillers required for these compositions produce cured elastomershaving extremely low tensile and tear strengths, elongations andpermanent set values. Elastically recoverable splices require tearstrengths of at least about 10 N/mm, and also good elongation values,e.g., at least about 400%, in order to expand radially as required toterminate or splice an electrical cable without fracture of one or morelayers. Further, electrical splices must be able to survive 30 days ofcurrent cycles without reaching temperatures above 165° C.; thereforethe splice and material made therefrom must meet a variety of severestandards; it must be electrically insulating, thermally conducting andphysically strong but flexible.

The current inventor has now discovered that a splice having an uniformcylindrical body, two opposing geometric cones capped by extended endseals where all layers are formed from a silicone, with the insulationbeing formed from a thermally conductive silicone elastomer, exhibitsexcellent electrical properties and physical properties over thesilicone splices previously available. Surprisingly, the splicemaintains a lower temperature during extreme emergency overloadconditions than the conductor beyond the splice.

SUMMARY OF THE INVENTION

The invention provides an elastically recoverable elastomeric splicecover suitable for use covering a joint connector in a cable conductor,said splice cover comprising a central body having two distal opposinggeometric cones capped by extended endseals, said body comprising threecontiguous layers,

a) a semiconductive outer shield layer,

b) an inner toroidal electrode disposed along a central axis having alength less than that of said body,

c) and interposed therebetween, an intermediate insulative layer havinga length equal to that of said body and said geometric cones, and

said outer shield layer and said electrode having been formed from athermally conductive silicone elastomer, all of said layers having beenformed from a silicone elastomer having a tear strength of at leastabout 15 N/mm, and an elongation of at least about 400%, said insulativelayer further including from about 10% to about 35% of an electricallyinsulative filler, said layer having a minimum thermal conductivity ofat least about 0.18 W/mK,

wherein after 30 alternating load current cycles said connector hasmaintained a temperature of about 10° C. cooler than said cableconductor beyond said splice.

In a preferred embodiment, the invention provides an elasticallyrecoverable elastomeric splice cover suitable for use covering a jointconnector in a cable conductor, said splice cover comprising a centralbody having two distal opposing geometric cones capped by extendedendseals, said body comprising three contiguous layers,

a) a semiconductive outer shield layer,

b) an inner toroidal electrode disposed along a central axis having alength less than that of said body, wherein said electrode has roundedends, including at least one undercut extending inwardly from at leastone of the ends and from the inner surface to a point intermediate theinner and the outer surface,

c) and interposed therebetween, an intermediate insulative layer havinga length equal to that of said body and said geometric cones, and

said outer shield layer and said electrode having been formed from athermally conductive silicone elastomer, all of said layers having beenformed from a silicone elastomer having a tear strength of at leastabout 20 N/mm, and an elongation of at least about 400%, said insulativelayer further including from about 10% to about 35% of an electricallyinsulative filler, said layer having a minimum thermal conductivity ofat least about 0.18 W/mK,

wherein after 30 alternating load current cycles said connector hasmaintained a temperature of about 10° C. cooler than said cableconductor beyond said splice.

The body of the splice cover may vary in shape for differentembodiments; it may maintain an annular shape, or it may have anincreased width for an area approximately adjacent to each end of theelectrode, as seen in FIG. 3. Likewise, the geometric cones and extendedendseals may vary in length.

As used herein, these terms have the following meanings.

1. The term "silicone elastomer" or "silicone rubber" means any of avariety of polyorganosiloxanes characterized by the presence of at leastone type of repeating unit having the formula

    R.sub.n SiO.sub.(4-n)/2

where R represents a monovalent hydrocarbon or substituted hydrocarbonradical and n is 1, 2, or 3.

2. The terms "elastically recoverable," "elastically shrinkable" and"cold shrinkable" are used interchangeably to mean that an article isshrinkable at temperatures of about -20° C. to about 50° C. without theaddition of heat.

3. The term "extended endseal" means an endseal of at least about 1 cm.in length.

4. The term "thermally conductive" when used to refer to a materialmeans that the material has good transfer of heat therethrough.

5. The term "tan delta" or "tan δ" refers to the electrical dissipationfactor.

6. The terms "permittivity" and "relative permittivity" is the ratio ofelectric flux generated by a field in a medium to that generated by thematerial in a vacuum.

7. The term "dielectric constant" is synonymous to relativepermittivity.

All parts, percents, and ratios herein are by weight unless otherwisespecifically stated.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a splice cover of the inventionpartially cut away to show parts in section. As this shows, the splicecover 10, is a unitary article having three layers; a semi-conductiveouter shield layer 11, an intermediate insulative layer 12, and an innersplice electrode, 13. The article has a central body, 14, two distalopposing geometric cones, 15, capped by extended endseals, 16. Thesplice cover is in a radially stretched state retained by a supportingcore, 19, typically formed from polyethylene or polypropylene. Thisembodiment of the core has a manual handle 18, which can be gripped andpulled to remove the core.

In FIG. 2, the splice cover 10, of FIG. 1 is shrunk onto a cable spliceor connection such that the electrode 13 extends along the connectorelement 21, and engages the insulation 22. The exposed conductors of thecable ends are interconnected by means of a sleeve-like connectorelement 21. The connector element 21 is crimped onto the conductors. Tothis purpose it is necessary to remove the cable's insulation 22. Theextended endseals 16 are in engagement with the cable insulation shieldlayer 23. It can be seen that, in this embodiment, the sleeve 10 whenshrunk has a constant outer diameter and also approximately a constantinner diameter with some small deformations occurring at the transitionsbetween layers.

FIG. 3 is another embodiment of the splice cover, again shown shrunkonto a cable splice or connection. In this embodiment, the splice body11 does not have a constant outer diameter, but rather has two annularridges 17 which approximately at the ends of the electrode 13, andterminate into the geometric cones 15.

DETAILED DESCRIPTION OF THE INVENTION

Splice covers of the invention are unitary multilayer articles which areuseful for protecting electrical junctions in medium voltage and highvoltage electrical cables. The cover comprises three layers; an innerelectrode, an intermediate insulative layer, and an outer semiconductiveshield layer.

All layers are made of silicone elastomers which have sufficientelasticity to be radially expanded and relaxed to be placed onto a cableconnection or termination.

Conductive silicone elastomers useful in splice covers of the inventioninclude those conductive silicones having minimum tear strengths of atleast about 20 N/mm, preferably at least about 30 N/mm, and elongationsof at least about 400%, preferably at least about 500%.

Silicone normally has a rather poor thermal conductivity unless fillersare heavily used. Various ceramic materials have been used as fillers torender such silicones thermally conductive; however, typically, suchhigh levels of the fillers have been required that the physicalproperties of the silicone have been comprised.

The thermally conductive silicone may be a liquid silicone or a gumsilicone; gum silicones are preferred for easy compounding andprocessability.

Preferred silicone elastomers for use in the insulative layer include,but are not limited to, liquid silicones available as Baysilone® LSRseries numbered 2030-2040, available from Bayer Corp., Elastosil®LR3013/40 to 3003/50, available from Wacker Silicones Corp., Silastic®9280-30 to -40 series from Dow Corning, "KE 1950-30 to 1950-40",available from Shincor Silicones Inc., and "LIM 6030-D1, and 6040-D1",available from General Electric Corp.; as well as gum siliconesavailable as Silastic® M2809 from Dow Corning, Elastosil® 4000/40through 4000/70 from Wacker Silicones Corporation, Tufel® I SE846, andTufel® II 94405, available from General Electric, "SVX-14007B",available from Shincor Silicones Inc. and "HVVP AC3537", available fromBayer Corp.

For use as the electrode layer, electrically conductive silicones suchas Elastosil® R573/50, available from Wacker Silicones and "KE-3611U",available from Shincor Silicones.

The silicone elastomers typically are provided as two components, whichmust be stored separately until ready for processing. The two componentsare then blended together and at a specified ratio, typically about 1:2to 2:1, preferably about 1:1, and molded into the desired shape.

Preferred silicones are platinum catalyzed silicones. The platinumcatalyst may be the individual platinum group metal and its compounds,e.g., extremely fine platinum powder on a carbon powder carrier,choroplatinic acid, platinum chelates, chloroplatinic acid-olefinproducts, and similar metal compounds of palladium rhodium, iridium,ruthenium and osmium. Useful amounts of catalyst are from about 0.01part to about 20 parts per 100 parts of organopolysiloxane.

Because the silicone elastomer is conductive, the electrode component ofthe splice does not require the addition of further conductive fillers.

The insulative layer of the splice necessarily contains a filler whichis electrically insulating yet thermally conductive. The insulativelayer contains from about 10% to about 35% of such electricallyinsulative filler, preferably from about 15% to about 30%. Usefulfillers are nonreinforcing fillers such as zinc oxide, iron oxide,aluminum oxide and hydrates thereof, commonly available under the namealumina and alumina trihydrate, e.g., Micral® 9401 from J.M. HuberCorp., the C-70 series, such as C-71, C-72, and the like, available fromAlcan Chemical, natural amorphous silica, such as that sold as "S MicronSilica", aluminum silicates, and the like. Such fillers may have surfacetreatments such as silanes, e.g., phenyltrimethoxysilane,vinyltrimethoxysilane, and the like.

Silicone polymers useful in splices of the invention may comprisefurther additives such as pigments or dyes for coloration of the spliceor a single layer thereof; such pigments include carbon black, pigmentRed 101, etc.; reinforcing silica fillers such as gels and aerosol,dispersants, flame retardants, and the like, so long as the amount oftype of additive does not exert an adverse effect on the physical orelectrical properties of the composition.

Splices of the invention have three contiguous layers; an innerelectrode, an intermediate insulative layer, and an outer semiconductiveshield layer.

The semi-conductive shield layer has a volume resistivity from about 30to about 270 ohm cm, preferably about 150 ohm cm; the shield layer isfrom about 1.25 to about 5.2 mm thickness for the elongate body portionof the splice, and forms the cones and elongate endseals at a thicknessof from about 2.5 mm to about 13 mm.

The inner or central electrode layer has a tubular or toroidal shapewith rounded or elliptical ends. The electrode has an average length offrom about 50% to about 90% of said body, disposed longitudinally in thecenter of the body. The volume resistivity of the electrode is fromabout 30 ohm cm to about 270 ohm cm, preferably from about 50 ohm cm toabout 100 ohm cm.

In a preferred embodiment, the electrode is an inner toroidal electrodehaving an inner surface adjacent the cable, an outer surface and twoends, the ends being rounded and including at least one undercutextending inwardly from at least one of the ends and from the innersurface to a point intermediate the inner and the outer surface.

The undercut may include an angled surface intersecting the rounded endand extending away from the intersection to the inner surface of theelectrode. The angled surface is preferably inclined at an angle of 45°with respect to the inner surface of the electrode.

In another embodiment, the angled surface may extend in a first surfacefrom the intersection with the rounded surface at a shallower angle,preferably 15°, and a second surface intersecting the first surface andinclined at a preferred angle of 45°.

The insulative layer is interposed between the electrode and thesemiconductive shield layer and also extends into the geometric cone.

Splices of the invention are provided in a radially expanded orstretched condition on a removable rigid core. Although any conventionaltype of core may be used, preferred articles of the invention areprovided on a rigid cylindrical core in the form of a helical coil, or aseries of coils, e.g., those disclosed in from U.S. Pat. Nos. 3,515,798,4,871,599 and 4,934,227, and 4,503,105, or the German patentspecification 37 15915. Adjacent convolutions of the support core areinterconnected in circumferential areas so that the coil may withstandthe inherent radial forces of the splice. A portion of the coil, i.e.,the removal strip, is led back through the coil and can be manuallygripped at one end of the splice. The convolutions will then separatesingly. By withdrawing the coil from the splice one helical convolutionat a time, the splice is allowed to radially shrink onto a connection orterminal, from one end to the other. Manual pulling of the removal stripprovides fully adequate force to unwind and remove the core, leaving thesplice tightly affixed to the cable.

The core may be made from a variety of materials, e.g., polyvinylchloride, polyethylene terephthalate, cellulose acetate butyrate, andthe like; the material need merely be a material which is sufficientlyrigid to support the splice, and allow manual removal of the entirecore, while being flexible enough to permit the required unwinding.

Splices of the invention have a long lifetime; e.g., at least about 20years. Long term reliability is a combination of the article design andthe long term stability of these materials. The geometry of the cableenables effective stress control with low danger of breakdowns andthermal runaway. It is critical to maintain an integral interfacebetween the cable insulation and splice insulation. The splice covers ofthe invention have the ability to be stored at 200%-250% expansion andthen perform at 20% expansion for many years. Because heating of thesplice is not required for installation, the connection can be operatedimmediately thereafter. The electric properties will be met in thenormally prevailing temperatures ranges of about -20° C. to about 130°C., without failure of the sealing against humidity and precipitation.

In order to form the splice, the silicone composition to be used foreach layer is mixed and cured, or vulcanized, at high temperatures. Thesilicone compositions may use the same or different silicones ormixtures thereof. However, the insulative layer must also have theelectrically insulative filler mixed therein. The electrode and thesemi-conductive shield layer are formed by molding, using anyconventional molding technique, but preferably injection molding.

After injection molding of the outer semi-conductive shield layer andthe electrode, the splice is assembled. While various conventionaltechniques are acceptable, the splice is preferably assembled using amandrel.

This assembly procedure involves providing a mandrel which has an areaof increased diameter somewhat larger than the diameter of theelectrode. The electrode is forced onto the mandrel. The outer shieldlayer is lid onto the mandrel over the electrode. A circular aperture isformed in the outer semiconductive shield layer approximately central tothe length of the splice body. Both ends of the outer shield layer arethen sealed, and an insulating compound injection nozzle is then forcefit into the aperture, and the entire assembly is placed into a moldwhich has the same shape as the exterior shield layer. The mold isclamped shut and insulating compound is injected through the nozzlebetween the outer shield layer and the electrode under high pressure.When the space is filled, the pressure of the insulating composition hasforced intimate contact with both the electrode, and the semiconductiveshield layer, as well as forcing the electrode to conform intimately tothe mandrel, and expanding the outer sleeve against the interior of themold. Thus, any voids are eliminated. Finally, the insulating compoundis cured by heating the mold to a temperature of from about 100° C. toabout 200° C., preferably at least about 175° C. for a period of fromabout 2 minutes to about 15 minutes.

The following examples are meant to be illustrative and are not intendedto limit the scope of the invention which is expressed solely by theclaims.

Test Methods

The following test methods are used in the examples.

    ______________________________________                                        Physical Properties                                                           Hardness             ASTM D 2240-86                                           Elongation           ASTM D 412-87                                            Tensile Strength     ASTM D 412-87                                            100% Modulus           "                                                      200% Modulus           "                                                      300% Modulus           "                                                      Tear Strength Die B  ASTM D 624-86                                            Tear Elongation Die B                                                                                "                                                      Tear Strength Die C    "                                                      Tear Elongation Die C                                                                                "                                                      Specific Gravity     ASTM D 792-86                                            Electrical Properties                                                         Alternating Cycle Test                                                                             IEEE 404                                                 A/C Withstand          "                                                      Dielectric Strength  ASTM-D-149-93                                            ______________________________________                                    

EXAMPLES Examples 1-7

Liquid silicone formulations were made using Baysilone® LSR 2030 (partsA and B), available from Bayer Corp., Baysilone® U10, a vinyl-terminatedpolydimethylsiloxane containing SiH groups, available from Bayer Corp.,a crosslinking agent Baysilone 430, also from Bayer Corp., and BaysiloneU catalyst. The formulation was mixed, and allowed to cure for 10minutes at 175° C. In Example 6, the indicated number of parts of C71FGalumina, available from Alcan Company was added directly to thesilicone; in Example 7, the alumina was added in a slurry. Theformulations are shown in detail in Table 1.

These formulations were then molded into 10 cm×0.2 cm slabs for 10minutes at 175° C. and post cured for 4 hours at 200° C. The sampleswere then tested for various physical characteristics. The formulationscontaining the aluminum have greater thermal conductivity; howeverExample 7 shows that the effect of adding the alumina in a slurry is areduction of modulus at all elongations measured.

                  TABLE I                                                         ______________________________________                                        Ingred.     Example No.                                                       (wt %)      1      2     3     4   5     6    7                               ______________________________________                                        LSR2030A    50     45    48.4  50  43.7  38.5 34.6                            LSR2030B    50     45    48.4  50  43.7  38.5 34.6                            C71FG Alumina                                                                             --     --    --    --  --    23.0 23.1                            U10 Oil     --     10    --    --  9.7   --   7.7                             Baysilone   --     --    3.2   --  2.9   --   --                              Crosslinking Agt. 430                                                         Pt Catalyst (ppm)                                                                         --     --    --     2  2     --   --                              ______________________________________                                    

                  TABLE II                                                        ______________________________________                                               1     2      3      4    5    6     7                                  ______________________________________                                        Thermal Cond.                                                                          0.142   *      *    *    *    0.191 0.180                            (W/m K)                                                                       Shore A  31      28     36   31   33   39    35                               Hardness                                                                      Permanent Set                                                                          1.4     2.5    1.0  1.2  <0.5 2.7   2.5                              (%)                                                                           Elong. at                                                                              749     757    708  843  646  683   653                              Break (%)                                                                     Tensile at                                                                             11.8    9.8    10.3 11.0 9.3  9.0   7.7                              Break (MPa)                                                                   Modulus at                                                                             .7      .5     1.0  .6   .9   1.1   .9                               100% (MPa)                                                                    Modulus at                                                                             1.4     1.2    2.2  1.2  2.1  2.2   2.2                              200% (MPa)                                                                    Modulus at                                                                             2.3     2.1    3.6  2.1  3.4  3.9   3.6                              300% (MPa)                                                                    ______________________________________                                         *Thermal conductivity is not affected by amount of catalyst, U10, and         crosslinker.                                                             

Example 8 and Comparative Example 9

Example 8 was made from 100 parts gum silicone; i.e., Silastic® M2809,available from Dow Corning, 30 parts C71FG alumina, available from AlcanCompany. The silicone, catalyst, and C71FG alumina were compounded andmixed using 2 roll mills, or dough mixers.

Example 6 is as described above, and Comparative Example 9 is theinsulation used in a commercial splice cover "QS2000LSR", available from3M Company. All materials were molded into slabs.

The three slabs were tested for physical, electrical, and thermalproperties, and the values are shown in Table 3. This table shows thatcompositions of the invention have improved tear strength (for Example8) over that of the commercial splice cover. Therefore, materials of theinvention exhibit an improved thermal conductivity without an attendantdecrease in physical properties.

Further, when Example 8 was tested using IEEE 404, 15 kV classrequirements, the connector temperature under the splice cover held atemperature of about 10° C. less than the conductor temperature (130°C.). A second sample identical to Example 8 was tested under the sameconditions, and the connector temperature was 17° C. less than theconductor temperature.

                  TABLE III                                                       ______________________________________                                                      Example 8                                                                             Example 6                                                                              Example C9                                     ______________________________________                                        Physical Property                                                             Hardness Shore A                                                                              50        39       31                                         Elongation (%)  690       683      749                                        Tensile Strength (MPa)                                                                        8.4       9.0      11.8                                       100% Modulus (MPa)                                                                            2.4       1.1      .7                                         200% Modulus (MPa)                                                                            3.5       2.4      1.4                                        300% Modulus (MPa)                                                                            4.2       3.8      2.3                                        Tear Strength (N/m)                                                                           40,300    --       20,000                                     Tear Elongation (%)                                                                           330       --       --                                         Permanent Set (22 hrs/100° C./                                                         5.2       2.7      1.4                                        100%)                                                                         Electrical Properties                                                         Dielectric Constant (500 V)                                                                   3.3       --       3.3                                        tan δ (500 V)                                                                           .0048     --       .0025                                      Volume Resistivity (500 V)                                                                    6 × 10.sup.14                                                                     --       3 × 10.sup.15                        Dielect. Stren. (kV/mm)                                                                       21.2      --       22.0                                       Thermal Properties                                                                            .242      .191     .167                                       Thermal Conductivity (W/m K)                                                  ______________________________________                                         commercial Product                                                       

What is claimed is:
 1. An elastically recoverable elastomeric splicecover suitable for use covering a joint connector in a cable conductor,said splice cover comprising a central body having two distal opposinggeometric cones capped by extended endseals, said body consistingessentially of three contiguous layers,a) a semiconductive outer shieldlayer, b) an inner toroidal electrode disposed along a central axishaving a length less than that of said body, c) and interposedtherebetween, an intermediate insulative layer having a length equal tothat of said body and said geometric cones, and said outer shield layerand said electrode having been formed from a thermally conductivesilicone elastomer, all of said layers having been formed from asilicone elastomer having a tear strength of at least about 15 N/mm, andan elongation of at least about 400%, said insulative layer furtherincluding from about 10% to about 35% of an electrically insulativethermally conductive filler, said insulative layer having a minimumthermal conductivity of at least about 0.18 W/mK,wherein after 30alternating load current cycles said connector has maintained atemperature of about 10° C. cooler than said cable conductor beyond saidsplice.
 2. An elastically recoverable splice cover according to claim 1wherein said silicone elastomer is selected from the group consisting ofliquid silicone elastomers and gum silicone elastomers, said elastomeralso having a permanent set of no more than about 5%.
 3. An elasticallyrecoverable splice cover according to claim 1 wherein said siliconeelastomer has a tear strength of at least 20 N/mm.
 4. An elasticallyrecoverable splice cover according to claim 1 wherein at least one ofsaid layers is formed from a blend of silicone elastomers.
 5. Anelastically recoverable splice cover according to claim 1 wherein saidinsulative layer includes from about 10% to about 30% of saidelectrically insulative, thermally conductive filler selected from thegroup consisting of zinc oxide, iron oxide, aluminum oxide and hydratesthereof.
 6. An elastically recoverable splice cover according to claim 1wherein said thermal conductivity is at least about 0.24 W/mK.
 7. Anelastically recoverable splice cover being suitable for use covering ajoint connector in a cable conductor, said splice cover comprising acentral body having two distal opposing geometric cones capped byextended endseals, said body comprising three contiguous layers,a) asemiconductive outer shield layer, b) an inner toroidal electrodedisposed along a central axis having a length less than that of saidbody, c) and interposed therebetween, an intermediate insulative layerhaving a length equal to that of said body and said geometric cones, andsaid outer shield layer and said electrode having been formed from athermally conductive silicone elastomer, all of said layers having beenformed from a silicone elastomer having a tear strength of at leastabout 15 N/mm, and an elongation of at least about 400%, said insulativelayer further including from about 10% to about 35% of an electricallyinsulative filler, said insulative layer having a minimum thermalconductivity of at least about 0.18 W/mK,wherein after 30 alternatingload current cycles said connector has maintained a temperature of about10° C. cooler than said cable conductor beyond said splice, wherein allof said layers are formed by injection molding.
 8. An elasticallyrecoverable splice cover according to claim 1 wherein said endseals havea length of at least about 1 cm.
 9. An elastically recoverable splicecover according to claim 1 wherein said electrode has a volumeresistivity of from about 30 to about 250 ohm-cm.
 10. An elasticallyrecoverable elastomeric splice cover suitable for use covering a jointconnector in a cable conductor, said splice cover comprising a centralbody having two distal opposing geometric cones capped by extendedendseals, said body comprising three contiguous layers,a) asemiconductive outer shield layer, b) an inner toroidal electrodedisposed along a central axis having a length less than that of saidbody, wherein said electrode has rounded ends, including at least oneundercut extending inwardly from at least one of the ends and from aninner surface to a point intermediate to said inner surface and an outersurface, c) and interposed therebetween, an intermediate insulativelayer having a length equal to that of said body and said geometriccones, and said outer shield layer and said electrode having been formedfrom a thermally conductive silicone elastomer, all of said layershaving been formed from a silicone elastomer having a tear strength ofat least about 20 N/mm, and an elongation of at least about 400%, saidinsulative layer further including from about 10% to about 35% of anelectrically insulative thermally conductive filler, said insulativelayer having a minimum thermal conductivity of at least about 0.18W/mK,wherein after 30 alternating load current cycles said connector hasmaintained a temperature of about 10° C. cooler than said cableconductor beyond said splice.
 11. An elastically recoverable splicecover according to claim 10 wherein said thermal conductivity is atleast about 0.24 W/mK.
 12. An elastically shrinkable splice coverwherein said cover is formed by injection molding, suitable for usecovering a joint connector in a cable conductor, said splice covercomprising a central body having two distal opposing geometric conescapped by extended endseals, said body comprising three contiguouslayers,a) a semiconductive outer shield layer, b) an inner toroidalelectrode disposed along a central axis having a length less than thatof said body, wherein said electrode has rounded ends, including atleast one undercut extending inwardly from at least one of the ends andfrom an inner surface to a point intermediate to said inner surface andan outer surface, c) and interposed therebetween, an intermediateinsulative layer having a length equal to that of said body and saidgeometric cones, and said outer shield layer and said electrode havingbeen formed from a thermally conductive silicone elastomer, all of saidlayers having been formed from a silicone elastomer having a tearstrength of at least about 20 N/mm, and an elongation of at least about400%, said insulative layer further including from about 10% to about35% of an electrically insulative filler, said insulative layer having aminimum thermal conductivity of at least about 0.18 W/mK,wherein after30 alternating load current cycles said connector has maintained atemperature of about 10° C. cooler than said cable conductor beyond saidsplice.
 13. An elastically recoverable splice cover according to claim10 wherein said insulative layer includes from about 10% to about 30% ofsaid electrically insulative, thermally conductive filler selected fromthe group consisting of zinc oxide, iron oxide, aluminum oxide andhydrates thereof.
 14. An elastically recoverable splice cover accordingto claim 13 wherein said thermally conductive filler is selected fromthe group consisting of aluminum oxide and hydrates thereof.
 15. Anelastically recoverable splice cover according to claim 1 wherein saidsplice body has an annular shape and a constant outer diameter.
 16. Anelastically recoverable splice cover according to claim 1 wherein saidsplice body has two annular ridges.